1. Field of the Invention
The present invention relates to a process for assembling a hybrid electrochemical system.
2. Description of Related Art
It is known, in the prior art, that a hybrid supercapacitor, combining the storage principles of a lithium ion secondary battery and of an electrochemical double-layer capacitor (EDLC), has a higher energy density, generally of the order of 7 Wh·kg−1, than a standard EDLC. A symmetrical cell of a standard EDLC is composed of two identical capacitive electrodes. The potential difference of such an uncharged cell is 0 V and it increases linearly with time during the galvanostatic charging of the cell. During the charging, the potential of the positive electrode increases linearly and the potential of the negative electrode decreases linearly.
During discharging, the cell voltage decreases linearly. Industrial symmetrical EDLCs operating in an organic medium usually have a nominal voltage of 2.7 V. A contrario, the electrode of lithium battery type is characterized by a virtually constant potential during the charging and discharging of the system. In order to increase the operating voltage of a supercapacitor, it is possible to replace the negative electrode of an EDLC with a carbon-based electrode of “lithium battery” type.
The main problems to be solved in this type of hybrid supercapacitor are the formation of the passivation layer and the intercalation/insertion of the lithium into the negative electrode. In a first step, the passivation of the negative electrode makes the formation possible, on this electrode, of an intermediate layer during a first specific charging cycle. In the presence of this passivation layer, the lithium ions are desolvated before being intercalated/inserted into the negative electrode. The presence of a well-formed passivation layer makes it possible to prevent the exfoliation of the negative carbon electrode during the cycling of the system. The lithium is intercalated/inserted into the negative electrode until a composition Li˜0.5C6 is achieved. Thus, the potential of the negative electrode remains relatively stable during the successive charges/discharges of the hybrid supercapacitor.
In the state of the art, different solutions are normally selected to produce the passivation layer and to intercalate/insert a sufficient amount of lithium ions into the negative electrode:
i) to use a source of lithium metal in order to prevent the electrolyte from being depleted, as described, for example, in patent application EP-A1-1 400 996;
ii) to carry out the ex situ intercalation/insertion of the lithium into the electrode active material, for example by reactive grinding;
iii) to saturate the surface functional groups of the activated positive carbon electrode by means of an electrolytic solution comprising lithium ions, for example by means of an aqueous LiOH solution, as provided for in patent application JP 2008-177263. The lithium present in the positive electrode subsequently makes it possible to carry out the intercalation/insertion into the negative electrode without depleting the electrolyte.
The disadvantage of the use of lithium metal is that it is particularly expensive and restrictive industrially. Furthermore, in the presence of organic solvent, the lithium metal can give rise to thermal runaway and can thus present safety problems.
The ex situ production of a compound for intercalation/insertion of the lithium is also problematic: the material subsequently has to be able to be handled for the purpose of the manufacture of the energy storage system. In point of fact, it is known that these materials are particularly reactive with regard to oxygen, water and nitrogen.
These three solutions are thus not economically and/or technically satisfactory.
There is also known, from the publication “High-energy density graphite/AC capacitor in organic electrolyte” (V. Khomenko, E. Raymundo-Pinero and F. Beguin), which appeared in the Journal of Power Sources, 177 (2008), 643-651, a process for assembling a hybrid supercapacitor, that is to say an electrochemical energy store system comprising, on the one hand, a negative electrode based on nonporous or only slightly porous carbon (for example graphite), said electrode being an electrode conventionally used as anode in lithium batteries, and, on the other hand, a positive electrode typically used in electrochemical double-layer capacitors, that is to say based on nanoporous carbon, in which the intercalation/insertion of the lithium at the negative electrode is carried out using the lithium salt occurring in the electrolyte. However, the cells used are laboratory cells providing a sufficiently great reservoir of electrolyte for the conductivity and the composition of the electrolyte to remain unchanged during the intercalation/insertion of the lithium into the negative electrode. In the case of more compact systems, the volume of electrolyte is limited and the intercalation/insertion of the lithium into the negative electrode depletes the electrolyte, which brings about a fall in the performance of the system.